Bourbaki

Lemma 8

Tag 0434, permanent. Cite it as [Bourbaki, Tag 0434].

Commutative Algebra, chapter I, § 2. Flat modules

(i) Every module admitting a finite presentation is finitely generated. (ii) If AA is a left Noetherian ring, every finitely generated left AA-module admits a finite presentation. (iii) Every finitely generated projective module admits a finite presentation.

Assertion (i) follows trivially from the definitions. If AA is left Noetherian and there exists a surjective homomorphism u:L0→Eu : L_0 \to E, where L0L_0 is a free left AA-module with a finite basis, the kernel RR of uu is finitely generated (Algebra, Chapter VIII, , no. 1, Proposition 1 and no. 3, Proposition 7), hence there is a surjective homomorphism u:L1→Ru : L_1 \to R, where L1L_1 is free and has a finite basis, and the exact sequence L1→vL0→uE→0L_1 \xrightarrow{v} L_0 \xrightarrow{u} E \to 0 is a finite presentation of EE; whence (ii)

Finally, suppose that EE is a finitely generated projective module; then it is a direct factor of a finitely generated free module L0L_0 (Algebra, Chapter II, §2, no. 2, Corollary to Proposition 4); the kernel RR of the surjective homomorphism L0→EL_0 \to E is then isomorphic to a quotient of L0L_0 and hence finitely generated and the proof is completed as for (ii).

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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